do_select() improvement ...

[Davide Libenzi]

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This patch can improve do_select() ==> select() performance due :

1) the load from the fd bitmap is done every __NFDBITS instead of every bit.
        If You look at the BITS() macro You'll see that it's not cheap in terms
        of memory operations

2) if a sequential hole of __NFDBITS is found it'll be skipped without doing a
        bit by bit check

*** select.orig.c       Thu Jun 21 08:52:04 2001
--- select.c    Thu Jun 21 12:09:25 2001
***************
*** 164,171 ****
--- 164,172 ----
  {
        poll_table table, *wait;
        int retval, i, off;
        long __timeout = *timeout;
+       unsigned long bits;
 
        read_lock(&current->files->file_lock);
        retval = max_select_fd(n, fds);
        read_unlock(&current->files->file_lock);
***************
*** 186,194 ****
                        unsigned long mask;
                        struct file *file;
 
                        off = i / __NFDBITS;
!                       if (!(bit & BITS(fds, off)))
                                continue;
                        file = fget(i);
                        mask = POLLNVAL;
                        if (file) {
--- 187,202 ----
                        unsigned long mask;
                        struct file *file;
 
                        off = i / __NFDBITS;
!                       if (!(i & (__NFDBITS - 1))) {
!                               bits = BITS(fds, off);
!                               if (!bits) {
!                                       i += __NFDBITS;
!                                       continue;
!                               }
!                       }
!                       if (!(bit & bits))
                                continue;
                        file = fget(i);
                        mask = POLLNVAL;
                        if (file) {
 
                                                                               
                                             

                                                                             
                                             

- Davide

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